1. Technical Field
This invention relates to an apparatus and method for transferring heat from a hot, contaminated, fluid stream to a second fluid stream and, for heating the second stream to a temperature approaching the temperature of the hot fluid stream. In addition, the invention relates to an apparatus and method for removing the majority of the contaminants from the hot fluid stream directly and also for reducing the temperature of the hot fluid stream sufficiently so that the contaminants behave as a dust and can be removed by conventional means.
2. Background Art
Exhaust gas from the combustion of coal and the exhaust gas from certain other processes, such as lime calcining, contain mixtures of contaminants including minerals and alkali salts which were originally combined with the fuel such as the coal, or the feed material, such as the lime, as it was mined from the earth. At temperatures up to 1700.degree. F., these chemically complex mixtures of contaminants generally behave as free-flowing solid particles. The contaminants settle on heat transfer surfaces such as boiler tubes, but can be removed by soot blowers. Their presence in the gas stream does, however, preclude the use of intricate, extended surface heat transfer devices.
As the temperature of the hot exhaust gas stream and hence the contaminant particles increases above 1700.degree. F. to 2000.degree. F., some of the constituents soften into sticky particles, and eventually the particles form a mass of liquid slag. At temperatures of 2600.degree. F. and above, some of the alkali salts become vaporized and are carried with the gas as a vapor. The heat transfer from a contaminated or dirty gas stream to another gas stream at these elevated temperatures is difficult because the sticky contaminants collect on heat transfer surfaces and foul the heat exchanger. In addition, the contaminants, in their liquid state, wet the heat exchanger surface and the alkali, sulfur, and chlorine, which the contaminants normally contain, cause hot corrosion of these surfaces. Normally, the solution to this problem is either (1) to cool the contaminated gas to a lower temperature prior to heat transfer by dilution, as taught in the case of a lime calciner by U.S. Pat. No. 3,998,929, issued to Leyshon, or (2) to transfer the heat to a surface well below the 1700.degree. F. to 2000.degree. F. temperature limits as, for example, in the case of a pulverized coal utility boiler. In either case, the benefit from the elevated temperature is lost.
In industrial processes that must operate at elevated temperatures of 2500.degree. F. and above, such as, for example, glass furnaces and blast furnaces, brick checkers are ued to absorb the heat from the exhaust gas and return it to the process by alternating the flow of hot exhaust gas and incoming air thereover. These simple heat transfer devices will operate with only a limited amount of contaminants in the hot exhaust stream. In addition, the crude brick checkers will not deliver an average preheat air temperature approaching the exhaust gas temperature. For example, a typical blast furnace checker absorbs heat from an exhaust stream at 2900.degree. F., but only delivers air preheated to 2300.degree. F.
Nowhere is the requirement to preheat incoming air more severe than in a magnetohydrodynamic (MHD) power plant. To maintain reasonable efficiencies, the incoming air must be preheated to 2500.degree. F. (with 2900.degree. F.-3200.degree. F. desired) from an exhaust gas stream at 3600.degree. F. The exhaust gas stream can contain, for example, 0.4% coal slag and 3% potassium sulfate, if high sulfur coal is burned. It is noted that potassium carbonate is added to the gas stream to provide ions which are necessary to generate electric power in MHD systems. The potassium combines with sulfur in the coal to form potassium sulfate in the exhaust gas stream. Corrosion of the refractory, surface deposition, and corrosion of boiler tubes by liquid potassium sulfate are the major potential problems. A successful MHD air preheater using exhaust gases is yet to be built.
Fluidized beds are well known in the art, and have been used in industry for many years for a variety of applications. Their first widespread use was for petroleum catalytic crackers during World War II. Since that time, they have been used in industry for ore roasting, lime calcining, drying, heat treating of metals, and, most recently, for the combustion of low grade solid fuel, including high sulfur coal.
One of the many advantageous features of fluid beds is the excellent heat transfer between the incoming gas and the media due to the large surface area of the media and the turbulent action of the bed. Likewise, the heat transfer from the media to a body submerged in the bed is also excellent. U.S. Pat. No. 3,912,002, issued to Elliot, teaches the use of a shallow fluid bed with finned tubes buried therein to make a compact heat exchanger. U.S. Pat. No. 3,982,901, issued to Steever, teaches the immersion of boiler tubes in a coal-burning fluid bed at 1700.degree. F. to produce steam. None of the above, however, teaches the use of the fluid bed to nullify the deleterious effects of the contaminant in the hot gas while simultaneously using the well known heat transfer features. In fact, these fluid beds would quickly become ineffective were contaminated hot gas directed therethrough.
Beds of granular media have been used to filter gases. For example, see U.S. Pat. No. 4,012,210, issued to Morris; U.S. Pat. No. 4,017,278 issued to Reese; and U.S. Pat. No. 3,940,237 issued to Gonzalez, which teach the use of a slowly moving packed bed to filter particulate from a gas, heat transfer not being a consideration. U.S. Pat. No. 3,953,190, issued to Lange, teaches the use of a slowly moving packed bed to filter the exhaust of a glass furnace and preheat the incoming charge, which charge is in fact the packed bed. U.S. Pat. No. 3,847,094, issued to Taeymans, et al, teaches the passage of the exhaust gas stream of an incinerator through a fluidized bed but only for the purpose of incinerating the unburned particles.